1. Technical Field of the Invention
This invention relates to viscous aqueous liquids and, more specifically, to the reduction of the viscosity of viscous aqueous liquids. In one aspect, the invention relates to the reduction of the viscosity of viscous aqueous liquids introduced into fractures formed in subsurface earth formations.
2. Description of the Prior Art and Problems Solved
It is known in the art to add a solvatable, organic polymeric material, such as a polysaccharide, to an aqueous liquid to form a liquid colloidal dispersion referred to as a sol. It is further known in the art to add a reactant to the sol which, it is believed, causes the dispersed colloidal particles in the sol to form connections to produce a product referred to as a gel. The reactant is conventionally referred to as a cross-linking agent, or, more simply, as a cross linker. The formation of the sol upon addition of the polysaccharide to the aqueous liquid is accompanied by an increase in the viscosity of the aqueous liquid. Addition of a cross linker to the sol causes a further increase in the viscosity of the aqueous liquid.
In that branch of the petroleum producing arts which features the recovery of hydrocarbons from a subsurface formation, a “sol” is conventionally referred to as a “gel” and the hydrophillic polymer is conventionally referred to as a “gelling agent.” The product produced by addition of the reactant to the “sol” is conventionally referred to as a cross-linked gel. The petroleum arts terminology is employed in this disclosure.
Petroleum hydrocarbons are known to occur in the pores of identified subterranean formations. Accordingly, hydrocarbons are recovered from the pore spaces through a borehole drilled from the surface of the earth which penetrates an identified formation. Petroleum flows from the pores through the formation to the borehole and then to the surface. For a variety of reasons the rate of flow of hydrocarbons through a formation can be unsatisfactorily low. The art has, thus, developed a number of stimulation processes for increasing the rate of flow. Hydraulic fracturing is one such stimulation process, and features the use of an aqueous liquid under pressure to form a crack in the formation through which formation fluids, such as liquid hydrocarbons, can flow toward the borehole.
In brief, hydraulic fracturing, referred to herein as fracturing, involves the intentional increase of hydraulic pressure within the borehole at the intersection of the borehole and the formation, i.e., the formation face. At some time during the increase of the pressure at the formation face the pressure becomes sufficiently high to overcome the mechanical ability of the formation to resist the applied pressure. At that point, called breakdown, a crack opens in the formation and the hydraulic fluid, i.e., the fracturing fluid, such as water, flows into the crack causing the crack to increase in width and extend into the formation. So long as sufficient pressure is maintained on the fluid it will operate to prevent the crack from closing.
The stated purpose of hydraulic fracturing is to increase the rate of flow of hydrocarbons from the formation within the formed fracture toward the borehole. It is, therefore, necessary to reduce the applied pressure within the fracture to permit the desired fluid flow. However, a reduction in applied pressure will not result in a sufficient rate of flow if the fracture closes when the applied pressure is reduced. The art has solved the closure problem by the step of placing a solid material, referred to in the art as a proppant, into the formed fracture. The proppant functions to mechanically hold, i.e., prop, the crack open upon reduction of applied pressure. The proppant is suspended in a gel or a cross-linked gel which transports the proppant to and places the proppant in the formed fracture.
In the performance of a fracturing process, it is preferred that the viscosity of the fracturing fluid be sufficiently high to suspend the proppant in the fluid while it is being transported to the formation, but low enough to minimize surface pumping pressure while pressure at the formation face is being increased. In contrast, at the moment when the fracture occurs, i.e., breakdown, it is ideally preferred that the viscosity of the fluid immediately increase to a maximum value as the crack widens and lengthens, and that the viscosity remain at the maximum value to thereby suspend the proppant in the fracture while applied pressure declines and the fracture closes on the proppant.
With applied pressure reduced, accompanied by closure of the fracture on the proppant, it is necessary to reduce the viscosity of the viscous aqueous liquid so that it can flow from the fracture to enable the hydrocarbons in the formation to flow in the fracture to the borehole. The art has developed, and continues to develop, materials and methods of using the materials which operate on the gel and cross-linked gel to reduce the viscosity of the viscous aqueous liquid.